1. Field of the Invention
The present invention relates to a movement control apparatus for an optical head, and particularly to a control apparatus capable of controlling moving velocity of the optical head during track accessing by detecting relative moving velocity of the optical head to an optical disk.
2. Description of the Prior Art
Conventionally, in an optical disk apparatus, an optical head including an object lens or the like moves in the radial direction of an optical disk, and the capability of highly accurate positioning of the optical head is required especially during track accessing. A technology for such positioning of the optical head is disclosed in, e.g., Tomio Yoshida, Takeo Ohta and Shunji Ohara, SPIE Vol. 329 Optical Disk Technology (1982), pp. 40-45 "Optical Video Recoder Using Tellurium Sub-Oxide Thin Film Disk" and in Teruo Murakami, Isao Hoshino and Masafumi Mori, SPIE Vol. 329 Optical Disk Technology (1982), pp. 25-32 "Optical Disk Memory System".
In order to control the movement of the optical head while accessing tracks of an optical disk, detection of the moving velocity of the optical disk itself is commonly needed. Is is known that a velocity detecting system which is conventionally used for a magnetic disk apparatus can be adapted to an optical disk apparatus for detecting such moving velocity.
FIG. 1 is a schematic block diagram showing a magnetic head control system of the above described conventional magnetic disk storage at the time of track accessing; and FIG. 2 is a schematic diagram of a velocity detection circuit included in the control system of FIG. 1. The circuit shown in FIGS. 1 and 2 is disclosed in e.g. Servo Technology Manual Vol. 2 (issued by New Technology Developing Center) which was published in Japan.
The structure of the magnetic head control system of the conventional magnetic disk storage will be described with reference to FIGS. 1 and 2. Referring to FIG. 1, a servo signal reproduced from a magnetic disk (not shown) by a magnetic head 1 is applied to a position detection circuit 2. Responding to this signal, this position detection circuit 2 generates a voltage which is in proportion to the displacement of the magnetic head 1 to be applied to a velocity detection circuit 3. On the other hand, the velocity of the magnetic head 1 is controlled by a voice coil 7, and a current which flows through this voice coil 7, i.e., a current obtained from a power amplifier 6 is also applied to the velocity detection circuit 3. The velocity detection circuit 3 generates, as will be described later, a velocity detection signal which represents moving velocity of the magnetic head 1 on the basis of outputs from the position detection circuit 2 and the power amplifier 6, and applies it to a velocity curve generation circuit 4. The velocity curve generation circuit 4 converts the applied velocity detection signal into an aimed velocity voltage and applies to a servo logic circuit 5. The servo logic circuit 5 generates an operation signal of the voice coil 7 in response to the output of the velocity curve generation circuit 4. This operation signal is amplified by the power amplifier 6 and then applied to the voice coil 7.
FIG. 2 shows the velocity detection circuit 3 of FIG. 1 in detail, in which the output of the position detection circuit 2 is applied to a terminal 8, and the output of the power amplifier 6 is applied to a terminal 9. A capacitor 10, resistors 11 and 12, and an operational amplifier 13 form a differentiating operational circuit 14, which generates a velocity signal by differentiating the output of the position detection circuit 2. The velocity signal output of the differentiating operational circuit 14 is applied to the input of a non-inverting operational circuit 15 and the input of an inverting operational circuit 16; the output of the non-inverting operational circuit 15 is selected by a switch 17 to be applied to a resistor 19; the output of the inverting operational circuit 16 is selected by a switch 18 to be applied to the resistor 19. On the other hand, the output of the power amplifier 6 applied to the terminal 9, i.e. the current which flows through the voice coil 7, is applied to one input of an operational amplifier 21 through a resistor 20. When a switch 22 is on, the operational amplifier 21 together with resistors 19 and 23 and a capacitor 24, form a low pass filter. When the switch 22 is off, the operational amplifier together with the resistor 19 and the capacitor 24 form an integrator. In addition, an offset adjusting resistor 25 is connected between the other input of the operational amplifier 21 and the ground potential. The output of the operational amplifier 21 is applied to an output terminal 26.
FIG. 3 is a diagram of waveforms for describing the operation of the circuit shown in FIGS. 1 and 2. First, the operation of the magnetic head control system of the conventional magnetic disk storage shown in FIG. 1 will be described with reference to FIG. 3. In the control system of FIG. 1, during track accessing, a servo signal detected by the magnetic head 1 is applied to the position detection circuit 2, which in turn generates a position signal S20 (FIG. 3) which is a voltage signal corresponding to the displacement of the magnetic head 1. The position signals 20 and the signal detected from the power amplifier 6 as a current flowing through the voice coil 7 are applied to the velocity detection circuit 3, and this velocity detection circuit 3 generates a voltage signal S28 (FIG. 3) which corresponds to the velocity, according to a method which will be described later. The signal S28 is converted into a velocity voltage aimed at moving the magnetic head 1 by means of the velocity curve generation circuit 4. The aimed velocity voltage is supplied to the power amplifier 6 through the servo logic circuit 5. A current for accelerating or decelerating the magnetic head 1 is applied to the voice coil 7 from the power amplifier 6, thereby controlling the moving velocity of the magnetic head 1 and enabling access to the aimed track.
The operation of the velocity detection circuit shown in FIG. 2 will be described with reference to FIG. 3. The position signal S20 is applied to the input terminal 8 of the velocity detection circuit 3 from the position detection circuit 2; a current signal S23 (FIG. 3) of the voice coil 7 is applied to the input terminal 9. Now, let us assume that a mode switching signal S22 (FIG. 3), which reaches a "H" level at the linear portion of the position signal S20 and reaches a "L" level at the non-linear portion of the signal S20, is applied to the switch 22. At the linear portion of the position signal S20, the mode switching signal S22 reaches the "H" level to turn the switch 22 ON, whereby the resistors 19 and 23, the capacitor 24 and the operational amplifier 21 form the low pass filter. The position signal S20 applied to the input terminal 8 is differentiated by the differentiating operational circuit 14, and outputted as the velocity signal S21. The velocity signal 21 is applied to the non-inverting operational circuit 15 and the inverting operational circuit 16. On this occasion, a signal S26 (FIG. 3), which reaches the "H" level when the polarity of the velocity signal S21 is positive, is applied to the switch 17, while a signal S27 (FIG. 3), which reaches the " H" level when the polarity of the signal S21 is negative, is applied to the switch 18. When the polarity of the signal S21 is positive, the switch 17 turns ON; when the polarity of the signal S21 is negative, the switch 18 turns OFF; and a signal S25 (FIG. 3) is applied to the resistor 19. On the other hand, at the non-linear portion of the position signal S20, the mode switching signal S22 reaches the "L" level to turn the switch 22 OFF, whereby the resistor 19, the capacitor 24 and the operational amplifier 21 form an integrator. The current signal S23 of the voice coil 7 which is applied to the input terminal 9, is applied to this integration circuit. Since the current flowing through the voice coil 7 corresponds to the carriage acceleration of the magnetic head 1, a velocity signal S24 (FIG. 3) can be obtained by the integration of this current. Accordingly, a velocity detection signal S28 of the magnetic head 1 is outputted from the output terminal 26.
Thus, a conventional magnetic disk storage is structured such that during track accessing, it detects the velocity signal by integrating current input which corresponds to the carriage acceleration of the magnetic head at the non-linear portion of the position signal, and that it detects the velocity signal by differentiating the position signal, then removing noise by the low pass filter, and absorbing the error in integration, at the linear portion of the position signal.
Now, it is known that track accessing can also be implemented in an optical disk apparatus by the above described control system of the magnetic disk storage. In this case, instead of the velocity control of the carriage of the magnetic head, the velocity control of a linear actuator for driving an optical head, which comprises an object lens, a tracking actuator for driving said object lens, and a photo detector, is required.
FIG. 4 is a schematic diagram showing a photo detector included in the above described optical head and the signal circuit thereof, which is disclosed in, e.g. Japanese Patent Laying-Open Gazette No. 134704/1977. In FIG. 4, an incident light spot through a tracking actuator (not shown) is received at a two-split photo detector 27; a subtraction amplifying circuit 28 outputs a difference signal (hereinafter referred to as a tracking error signal) of each of the outputs of the two-split photo detector 27; an addition amplifying circuit 29 outputs a sum signal (hereinafter referred to as an RF signal) of each of the outputs of the two-split photo detector 27.
FIGS. 5A and 5B show a cross section of an optical disk (a), a waveform of the tracking error signal (b) at the formation of a light spot thereon, and a waveform of the RF signal (c) on the same occasion. More particularly, referring to FIGS. 5A(a) and 5B(a), there are a number of guiding grooves 31 on the optical disk 30 forming concentric circles or a spiral and, in addition, a pit 32 is formed in a part of the guiding grooves 31, the depth of the pit 32 being deeper than that of the guiding groove 31.
In order to control the velocity of the optical head during track accessing of such optical disk, the difference signal of the two-split photo detector 27, i.e. a tracking error signal S29 (FIGS. 5A(b) and 5B(b)) of the push-pull type outputted from the subtraction amplifier circuit 28 is used as the position signal of the optical head. Namely, the tracking error signal S29 (the position signal of the optical head) and a current signal (the acceleration signal of the optical head) of a linear actuator (not shown) are applied to the circuit which corresponds to the velocity detection circuit 3 of FIG. 1, and the moving velocity of the optical head is detected by the differentiation of the tracking error signal and by the integration of the current signal of the linear actuator. On the basis of the velocity signal detected in the above described manner, a coil current of the linear actuator is controlled by the operation of various circuits corresponding to the velocity curve generation circuit 4, servo logic circuit 5 and the power amplifier 6 in FIG. 1, whereby the velocity control of the linear actuator is accomplished.
However, in case of track accessing according to the above described manner, the tracking error signal S29 becomes almost 0 at the pit 32 in which data is stored as shown in FIG. 5B. Accordingly, the differentiation of the velocity detection circuit brings about a velocity detection signal of 0, so that a normal velocity signal cannot be detected. Namely, the conventional control system includes a problem that the velocity control of the linear actuator of the optical disk apparatus cannot be carried out smoothly.
Another control system of the optical head during track accessing for a conventional optical disk apparatus is disclosed at the National Conference 1210 of the Institute of Electronics and Communication Engineers of Japan, 1983, entitled "Track Access System for an Optical Disk". FIG. 6 is a block diagram of such control system. The structure of the control system of FIG. 6 will be hereinafter described.
Referring to FIG. 6, a number of track grooves are formed on the surface of an optical disk 33 forming concentric circles or a spiral, and memory tracks comprised of high density pit lines are formed in the track grooves. The optical disk 33 is attached to a spindle 34 and revolved by a disk motor 35. The revolution of the disk motor 35 is controlled by a disk motor drive control circuit 36. A light spot is formed on the surface of the optical disk 33 by an optical head 37, which moves in the radial direction of the optical disk 33. Light radiated from a light source 38 such as a semiconductor laser passes through a collimator lens 39, a polarizing beam splitter 40, a .lambda./4 plate 41, an optical path changing mirror 42 and an object lens 43 to be focused on the surface of the optical disk 33 to form a light spot. In order to position the light spot precisely on the memory track of the optical disk 33, the object lens 43 is moved slightly in the radial direction of the optical disk 33 by a tracking actuator 44. The reflected light from the optical disk 33 is detected by a two-split or four-split photo detector 45. The above described light source 38, collimator lens 39, polarizing beam splitter 40, .lambda./4 plate 41, optical path changing mirror 42, object lens 43, tacking actuator 44 and photo detector 45 are put in a frame 46 forming the optical head 37. The output of the two-split or four-split photo detector 45 is applied to a known addition-subtraction amplifying circuit 47. The addition subtraction amplifying circuit 47 adds the outputs of the photo detector 45 to output a sum signal (RF signal) as an information signal, while it subtracts the outputs of the photo detector 45 to output a difference signal as a tracking error signal. The tracking error signal is generated according to a well known push-pull system. However, it may be other systems such as DPD system or the like. The tracking actuator drive control circuit 48 drives the tracking actuator 44 in response to the tracking error signal from the addition-subtraction amplifying circuit 47. In addition, the optical head 37 itself is moved in the radial direction of the optical disk 33 by the linear actuator 49. The linear actuator 49 is drive controlled by a linear actuator drive control circuit 50. A track traversing detection circuit 51 detects the number of tracks which the optical head 37 traversed in response to the output of the addition-subtraction amplifying circuit 47. A velocity detector 52 detects the absolute velocity of the optical head 37. An acceleration detector 53 detects the acceleration on the basis of the velocity detection signal from the velocity detector 52. An internal signal of the tracking actuator, drive control circuit 48, an internal signal of the linear actuator drive control circuit 50, and a revolution phase pulse of the optical disk 33 which is sent from the disk motor drive control circuit 36 are applied to a waveform memory circuit 54. A micro-CPU interface 55 is provided as an interface for a micro-processor.
The operation of the control system shown in FIG. 6 will be hereinafter described. First, the disk motor 35 is activated by the disk motor drive control circuit 36 and the optical disk 33 begins to rotate. After the steady number of revolution is attained, the optical head 37 forms a light spot on the optical disk 33 in a well known manner, and it receives the reflected light at the photo detector 45. Then, the tracking actuator drive control circuit 48 operates according to the tracking error signal detected by the addition subtraction amplifying circuit 47 to drive the tracking actuator 44, and the object lens 43 is moved so that the light beam spot follows the center of the track on the optical disk 33. On this occasion, the tracking actuator drive signal from the tracking actuator drive control circuit 48 is also applied to the linear actuator drive control circuit 50. The linear actuator drive control circuit 50 drives the linear actuator 49 at low frequency to move the optical head 37 in the radial direction of the optical disk 33 thereby reducing the movement of the tracking actuator 44.
During the track accessing, the number of tracks on the optical disk 33 which the light spot traversed is counted by the track traversing detection circuit 51 and, in addition, the moving velocity of the optical head 37 which is detected by the velocity detector 52 is applied to the linear actuator drive control circuit 50, whereby the velocity is controlled such that the moving velocity of the light spot approaches 0 as the light spot approaches the target.
In addition, during the movement of the optical head 37, the acceleration of the optical head is detected by the acceleration detection circuit 53 on the basis of the moving velocity of the optical head 37 which is detected by the velocity detection circuit 52. The acceleration signal is applied to the tracking actuator 44 through the tracking actuator drive control circuit 48. In response to this signal, the tracking actuator 44 equivalently compensates the inertia force of the object lens 43 which is gathered on the tracking actuator 44 during the acceleration or deceleration of the optical head 37, thereby preventing the vibration of the tracking actuator 44.
At the tracking mode, where the light spot is following the center of the track on the optical disk 33, the internal signals of the tracking actuator drive control circuit 48 and the linear actuator drive control circuit 50 are synchronized with the revolution phase pulse of the optical disk 33 from the disk motor drive control circuit 36 to be stored in the waveform memory circuit 54. During track accessing, the stored signal is applied to the tracking actuator 44 and the linear actuator 49 through respective drive control circuits 48 and 49, thereby reducing the relative eccentricity between the light spot formed by the optical head 37 and the optical disk 33.
In such a control system, the absolute velocity of the optical head 37 detected by the velocity detection circuit 52 is controlled. Accordingly, even if the moving velocity of the optical head 37 at the time when the light spot enters the track aimed at is controlled to be 0, the relative velocity of the light spot to the track on the optical disk 33 does not become 0 due to the eccentricity of the optical disk 33 or to the vibration of the tracking actuator 44, causing difficulty in counting the number of traversed tracks, or deterioration of the drawing operation of the tracking control system at the aimed track. Consequently, the waveform memory circuit 54 for compensating eccentric component, the micro-CPU interface 55 for controlling this circuit, or the acceleration detection circuit 53 for preventing vibration of the tracking actuator 44, must be provided, causing a problem that the structure of the control system becomes complicated.
In addition, since the non-cyclic velocity of the track vibration or the disturbance velocity which is unexpectedly added to the optical disk 33 cannot be detected by the velocity detection circuit 52, these factors cannot be controlled by the linear actuator drive control circuit 50 during track accessing, resulting in another problem that the drawing operation of the tracking control system at the track aimed at becomes unsuccessful.